Well, looks like you have all three alliances wrapped up (United for Star- US, Lufthansa, Singapore, etc; AA for One World-BA, etc; and DL for Skyteam- CO, NW, AirfranceKLM, Alitalia, KE, etc) and a LCC (Indy air) so you're all set in terms of number of FF accounts.
From CVG, you could focus on Delta if you don't like connections as DL will give you the most options.
But at any rate, AA, UA and DL all have some form of elite-status fasttrack or challenge, so if you plan to fly something like 10,000+ miles not on extremely cheapie fares in the next 3 months, check out their respective threads for info. Good luck!
Oh yeah, one thing I like about DL is free unlimited upgrades (when available) for elite members. But all four carriers are great...choose based on route schedule I suppose.